Nanocomposites are compositions that can address many of the challenges currently presented by automotive plastics and composites needs. These materials offer a variety of desirable properties including: low coefficient of thermal expansion, high heat deflection temperatures, lightweight, improved scratch resistance, and good surface appearance. Nanocomposite compositions are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers. These materials can be used in structural, semi-structural, high heat underhood, and Class A automotive components, among others. In other words these nanocomposites are compositions in which small particles are dispersed in the plastic matrix.
Reinforced plastic materials are continually finding new uses in automotive components. These materials have certain advantages over metals which include higher impact loads before deformation, lighter weight, increased design flexibility, and corrosion resistance. Automotive structural applications have traditionally been made from continuous glass mat composites and highly filled plastic materials such as sheet molding compound (“SMC”) where the polymeric component can be as little as 15% by weight. Both SMC and glass mat composite materials (“GMT”), however, are still relatively high in density.
Automobile trim and semi-structural components, on the other hand, are commonly fabricated from injection moldable thermoplastics and thermosets. These lighter weight composites, such as short fiber and mineral filled thermoplastics, could be substituted for metals or SMC and GMT composites in the same applications if their mechanical properties could meet the more stringent requirements. Virtually all bumper fascias and air intake manifolds have transitioned from metallic materials to plastics. As new plastic-based materials are developed, the transition will also encompass both more structural components, as well as Class A body panels and high heat under hood applications.
Injection moldable thermoplastics have long been mechanically reinforced by the addition of particulate and fiber fillers in order to improve mechanical properties such as stiffness, dimensional stability, and temperature resistance. Typical fillers include chopped glass fiber and talc, which are added at filler loadings of 20-40% in order to obtain significant mechanical reinforcement. At these loading levels, however, low temperature impact performance and material toughness are usually sacrificed. Polymer-silicate nanocomposite materials, in other words compositions in which the silicate is dispersed as very small particles, can address these issues.
Polymer-layered silicate nanocomposites normally incorporate a layered clay mineral filler in a polymer matrix. Layered silicates are made up of several hundred thin platelet layers stacked into an orderly packet known as a tactoid. Each of these platelets is characterized by large aspect ratio (diameter/thickness on the order of 100-1000). Accordingly, when the clay is dispersed homogeneously and exfoliated as individual platelets throughout the polymer matrix, dramatic increases in strength, flexural and Young's modulus, and heat distortion temperature are observed at very low filler loadings (<10% by weight) because of the large surface area contact between polymer and filler.
In some cases, it is necessary to include both platelet-type nanoparticles and other fillers to achieve desired characteristics. For example, U.S. Pat. No. 7,138,453 discloses dispersing platelet-type nanoparticles in a thermoplastic polyester and then melt kneading the resulting nanocomposite with reinforcing fibrous fillers, e.g., glass fiber, carbon fiber, aramid fiber, silicon carbide fiber, alumina fiber and boron fiber, whiskers such as silicon carbide whisker, silicon nitride whicker, magnesium oxide whisker, potassium titanate whisker and alunimo borate whisker, and needle crystals such as wollastonite, zonotolite, PMF, plaster fiber, dawsonite, MOS, phosphate fiber and sepiolite. For both processing capability and reinforcing efficacy, the reinforcing fibers are 2 to 20 micrometers in diameter (U.S. Pat. No. 7,138,453, col. 5, line 45 through col. 6, line 7; Examples 18 through 24).
Attempts to generate nanocomposites, or compositions containing nanosized particles dispersed in a thermoplastic polyester matrix, have been only marginally successful. It is desirable to disperse and exfoliate clays in polyesters both for automotive applications and to enhance barrier properties, for example, in packaging applications.
One route to preparing nanocomposite compositions is exfoliation through polymerization. This approach typically involves dispersing the nanofiller, usually a smectite like a montmorillonite, in one or more of the monomers and subsequently forming the polymer around the dispersion. One of the keys to successfully exfoliating the clay with this process involves selecting the proper intercalating agent. The interaction between the intercalating agent and the monomer must be sufficiently strong so that it is capable of driving the monomer into the galleries of the clay. Therefore, this process requires the use of an intercalating agent and as such introduces the same thermal stability issues described above.
Current literature typically teaches against the use of in situ polymerization approach for the preparation of clay nanocomposite compositions. For example, Matayabas et al. found that polymers prepared with organically modified clays did not exhibit any increase in the basal spacing of the clays after polymerization and no new basal spacings occurred during the polymerization. After transesterification, no individual platelets were identified. The formation of the individual platelets occurred during the polycondensation step of the polymerization process (J. C. Matabayas, Jr. et al, “Nanocomposite Technology For Enhancing The Gas Barrier,” in Polymer Clay Nanocomposites, T. J. Pinnavia, G. W. Beall eds., Wiley: New York, (2000) 218-222).
Another route employed in the preparation of polyester-based nanocomposite compositions is the use of another polymer such as poly(vinyl pyrrolidone) to facilitate the exfoliation of the clay into the polymer matrix. Nanocor® Inc. (Nanocor® Inc. is a wholly owned subsidiary of AMCOL International Corporation, Arlington Heights, Ill.) and Eastman Chemical Company (Kingsport, Tenn.) have both employed this approach in the preparation of polyester-based nanocomposites for use in applications that require materials with excellent barrier properties and mechanical properties (see, e.g., U.S. Pat. No. 5,698,624 to Nanocor® and PCT Int. Appl. WO 99/03914 to Eastman Chemical). However, this approach typically uses a solution based process that allows the clay and polymer to interact and increase the basal spacing on the clays. The solvent is subsequently removed under vacuum yielding an intercalated smectic clay system. The materials are then melt compounded with the desired polymer matrix (typically PET), extruded, and pelletized. This approach suffers from the requirement to use a large amount of solvent. For example, the polymer and clay represent only a small weight percent of the intercalation solution. (Trexler Jr., J. W., Piner, R. L., Turner, S. R. and Barbee, R. B. PCT Int. Appl. WO 99/03914). Furthermore, the introduction of a polymer (e.g., poly(vinyl pyrrolidone)) at the interface between the polyester and the clay filler alters the interaction between the polyester matrix and the nanoclay filler particles.
Another process for making thermoplastic polyester nanocomposites, using untreated “layered phyllosilicate” (i.e., platelet nanoclays), is disclosed in U.S. Pat. No. 7,138,453, comprising preparing a dispersion containing layered phyllosilicate and water, adding the dispersion continuously or successively to a component having low polymerization degree of the thermoplastic polyester resin at a rate of 0.01 to 10.0 parts by weight per minute based on 100 parts by weight of the component having low polymerization degree of the thermoplastic polyester resin; and polymerizing the thermoplastic polyester.
Thermoplastic polyester compositions in general are important items of commerce, being used for fibers, molded and extruded parts, foams, and other uses. Many of these polyesters are semicrystalline, such that part of the polyester is in a crystalline form in the end use part. In semicrystalline polymers in general, part of the polymer is present in an amorphous (glassy) form, and part of the polymer is present as crystallites, usually distributed throughout the polymer. In most instances, it is preferred that polyesters which can crystallize be used in the semicrystalline form, and often it is helpful or necessary that the polyester crystallize relatively rapidly for the purpose of forming the final part.
For example, in injection molding of thermoplastics, the molten polymer is injected into a mold and rapidly cooled until it is solid. The mold is then opened and solid part is ejected from the mold. If the part is not solid and/or deforms easily upon ejection from the mold, it may be deformed and thereby rendered useless. An important facet in obtaining a relatively strong part from semicrystalline polyesters is that they be (at least partially) crystallized when they are removed from the mold. However, some semicrystalline polyesters crystallize very slowly, so they would have to be in the mold a long time to allow them to be demolded without significant deformation. This would lead to long molding cycles, which is economically highly undesirable.
To solve this problem, so-called “crystallization packages” or “crystallization initiator systems” have been developed for slow crystallizing polyesters. These packages provide much faster crystallization initiation and/or a higher crystallization rate and/or a lower crystallization temperature. For example poly(ethylene terephthalate) (PET) is a slow crystallizing polyester, and by itself is usually unsuitable for injection molding because of the very long molding cycles and/or high mold temperatures needed. However crystallization packages have been developed for this polyester, making it suitable for injection molding and other forming processes. A typical crystallization package for PET is a sodium ion source such as a sodium or a sodium salt of a carboxylate containing polymer and a small amount of plasticizer for the PET; see for instance U.S. Pat. RE32,334. While not all polyesters are slow crystallizing, faster crystallization may lead to shorter melt processing cycle times which are more desirable.
For the reasons set forth above, there exists a need for an improved process for dispersing and exfoliating filler material in a polyester matrix.